Lithography for semiconductor manufacturing is constantly changing to shorter wavelengths in order to meet the need to print smaller features. Currently the industry is about to begin the transition from the current DUV (Deep Ultra Violet, i.e., 193 nm) lithography that provides its high-end manufacturing capability to EUV radiation (Extreme UV, i.e., 13.4 nm). The development of special EUV masks (reticles) for EUV lithography is now at final research and development stages.
Among other challenges, the following modifications in the design of the EUV masks are considered and implemented:
A. Different from transparent DUV masks, EUV masks have a metallic layer on the back side. The upper side of the EUV mask is reflective and during the lithographic process this upper side is illuminated by EUV radiation.
B. Different from DUV masks, EUV masks have no pellicle protective cover.
C. Implementation of EUV radiation in lithography requires vacuum during exposure, which will mechanically distort any thin pellicle protective cover mounted on the mask.
D. Cleaning requirements of EUV masks are much more demanding than for DUV masks, requiring (1) special carriers for carrying the EUV masks, and (2) vacuum at all stages where the EUV masks are exposed to EUV. In other worlds a tolerable cleanliness level for an EUV mask (especially the upper side of the EUV mask) is much higher than the cleanliness level maintained in scanners of DUV inspection tools.
A standard EUV pod (compliant with SEMI standard E152-0709) includes an inner pod receiving the EUV mask, placed inside of an external pod. The inner pod is made out of a durable, opaque material that prevents DUV inspection of the EUV mask while the EUV mask is within the EUV pod. The outer carrier is suitable for transporting the EUV masks between various production stations.
The following publications relate to DUV/EUV lithography and DUV/EUV mask handling: U.S. Pat. Nos. 6,239,863; 6,364,595; 6,492,067; 6,646,720; 6,734,445; 6,906,783; 6,912,043; 7,042,554; 7,209,220; 7,304,720; 7,413,586; 7,453,549; 7,477,358.
Mask inspection, which is part of the production flow of manufacturing semiconductor devices, is also required to transit from inspection of DUV masks to EUV masks. Among other approaches, the implementation of commercially available mask inspection technologies, which are based on DUV 193 nm illumination, is researched.
Existing 193 nm mask inspection tools (such as Aera2™ commercially available from Applied Materials™ Inc.) have the optical capability and resolution to support EUV inspection. However, these tools were not designed to address the challenges of handling the new EUV masks, for example, supporting the much higher mask environmental cleanliness requirements associated with EUV masks.